The present invention relates to eyeglass (ophthalmic) lenses with progressive surfaces. A surface of such a lens has an average surface refraction value ##EQU1## which changes continuously in any direction, where n is index of refraction and R.sub.1 and R.sub.2 are main curvature radii. In known lenses of this type according to French Pat. No. 1,544,799; GDR Pat. No. 71,210; French Pat. No. 2,058,499; German Offenlegungsschrift No. 2,439,127; German Auslegeschrift No. 2,044,639; and Austrian Pat. No. 322,238, progressive surfaces are formed by families of circles or conic sections, which intersect the main meridian at right angles. The main meridian is thus preferably, but not necessarily (ref: French Pat. No. 1,509,090) a plane curve, the curvature of which increases continuously from the far-sight field to the near-sight field and in which both main curvatures are of equal magnitude at each point on the progressive surface. This main meridian thus forms a so-called "umbilical line".
The present invention is related to the fact that the determining image properties of the progressive surface of a lens for the eyeglass wearer are determined by the average surface refraction value ##STR1## and the surface astigmatism ##EQU2## where r.sub.1 and r.sub.2 are main curvature radii, at all points on the progressive surface. All of the image properties taken together, such as sharpness of image and refraction accurateness of correction, and all characteristics by wearing, such as static and dynamic distortion and hibituation depend casually on these two values. The surface is thus clearly determined by giving the surface refraction value along an umbilical line and a family of lines of equal average surface refraction value intersecting the umbilical line.
In a desirable progressive surface, the surface refraction value D (on both sides of the umbilical line of average surface refraction value D) should increase from top to bottom not only along the umbilical line, but also uniformly around the periphery, with approximately the same gradient. According to a statement (theorem) by Minkwitz (Optica Acta, 10, 1963, 223), however, next the umbilical line the surface astigmatism .DELTA.D at a right angle to the umbilical line first increases twice, as the surface refraction value along this umbilical line changes. In addition at a greater distance from the umbilical line, the surface astigmatism is dependent on the course of lines of equal average surface refraction value and vice versa. A desirable progressive surface should now have on both sides along the main meridian either no surface astigmatism or only a slight one (e.g.&lt;0.25 diopters). The far sight field, i.e, the section essentially above the horizontal through the so-called optical far reference point B.sub.F, should have a constant average surface reference value and a small surface astigmatism (e.g..ltoreq.1.0 diopters) which is physiologically not troublesome, and which perhaps disappears far in the periphery. Accordingly, the same is true for the near sight section below the horizontal through the so-called optical near reference point B.sub.N.
The known progressive surfaces only fulfill the above requirements very inadequately, or not at all, since they are given either from a purely mathematical viewpoint of a sufficiently simple geometric law of formation (see, e.g. French Pat. Nos. 2,058,499, 1,544,799, German Offenlegungsschrift No. 2,439,127) or by the manufacturing process (see, e.g. French Pat. No. 1,095,375). The necessarily resulting image properties must be taken into account. If lines D=const. are calculated by this surface, sections with irregular such lines (D=const.) are encountered peripherally in the progression zone. This means, in part, that there is a considerable deviation from the refraction accurateness of the glasses in this visual regions. The eyeglass wearer, for example, finds in the progression zone of glasses (according to German OS No. 2,439,127 and French Pat. No. 2,058,499) regions with a more intense negative optical effect than in the far sight region; thus the glass is under-corrected for close vision and also under-corrected for distance vision in the case of hyperopia while over-corrected in the case of myopia. In addition, in a surface in which the lines of intersection are "evolutive conic sections" perpendicular to the main meridian (French Patent 2,058,499), lines of equal average surface refraction value are curved upward and incorporated far into the far sight field. In this way hyperopia is over-corrected and myopia, under-corrected. This sort of erroneous correction causes an image sharpness which, e.g., can have dangerous consequences when the person is in traffic, if a shortsightedness increased by 1 diopter or more through the wide opened pupil remains uncorrected.
Since the distribution of the surface astigmatism over the surface depends on the course of the lines of equal average surface refraction value and on the increase in curvature of the umbilical line, the known embodiments of progressive surfaces have considerable imperfections.
It is an important object of the invention to create the progressive surface from the physiological-optical standpoint so that the image properties and characteristics by wearing given above are improved.